Motor vehicles, and especially heavy motor vehicles such as trucks and buses, are affected by gravity on downhill gradients in such a way that their speed increases. For this reason, many heavy vehicles are equipped with a downhill speed control system, which is arranged to restrict the speed of the vehicle so that it does not exceed a predetermined value.
A set speed vset_dhsc for the downhill speed control system is set by the driver of the vehicle for example. Then the downhill speed control system ensures that this set speed vset_dhsc is not exceeded by the control of one or more brake devices in the vehicle. This is carried out against a reference speed vref_dhsc for the downhill speed control system, in which the reference speed vref_dhsc depends on the set speed vset_dhsc. The reference speed vref_dhsc often corresponds to the set speed vset_dhsc, except for an example when ramping up the setpoint of the downhill speed control system speed to increase comfort.
Braking devices are designed to create a brake torque which acts to reduce the speed of the vehicle, for example by reducing the speed of rotation of the wheelset providing the vehicle with propulsion. The braking devices can comprise one or more auxiliary brakes, such as an engine brake, exhaust brake, electromagnetic retarder and hydraulic retarder. Auxiliary brakes are useful for moderate deceleration and for downhill cruise control. This means that the ordinary wheel brakes are not worn and overheated unnecessarily.
For example, the set speed vset_dhsc can be set by the driver through some type of input, for example by pressing a button, turning a knob, activating a lever, input to a screen, activating a brake pedal, or through some other type of input familiar to experts in the field.
There are several different types of downhill speed control systems. One such type is a manual downhill speed control system, which can be utilised when no cruise control function is activated. Here the driver determines the set speed vset_dhsc through some type of button input, for example using a steering wheel button, which is then used to regulate one or more auxiliary brakes, for example an exhaust brake and retarder.
Another type of downhill speed control system is an automatic downhill speed control system. Here the set speed vset_dhsc is set using a brake pedal or the like, in such a way that the set speed vset_dhsc is set to a value corresponding to the actual speed vact at which the vehicle is travelling when the brake pedal is released after having been applied.
Another type of downhill speed control system is an offset downhill speed control system. Here the set speed vset_dhsc for the downhill speed control system is set as an offset to the set speed vset_cc for a cruise control system in the vehicle. If the offset downhill speed control system is deployed when a set speed vset_cc for a cruise control has been selected by the driver, the set speed vset_dhsc for the downhill speed control system will be related to the set speed vset_cc for the cruise control. In other words, the set speed vset_dhsc for the downhill speed control system is the same as the set speed for the cruise control system vset_cc plus an offset, vset_dhsc=vset_cc+ offset. For example this offset value can be between 3 km/h and 15 km/h. In other words, the function of the downhill speed control system is to prevent the speed of the vehicle exceeding a speed which is for example 3 km/h higher than the set speed vset_cc for the cruise control system if the offset has been selected as 3 km/h.